Add `register-node` command to pre-register nodes with Razor

In order to make brownfield deployments of Razor easier we allow users to
register nodes explicitly.  This command allows you to perform the same
registration that would happen when a new node checked in, ahead of time.

In order for this to be effective the hw_info must contain enough information
that the node can successfully be matched during the iPXE boot phase.

If the node matches an existing node, it will be treated as a request for
registration of the same node, and the existing node will be returned.

https://tickets.puppetlabs.com/browse/RAZOR-151

Signed-off-by: Daniel Pittman <daniel@rimspace.net>